The ontogeny of mammalian exocrine pancreatic function and the controls regulating this process will be elucidated by studies of the pancreas from the immature rat. The newborn rat pancreas, although structurally mature, is unresponsive to secretagogues. The development of secretory function will be characterized using a preparation of isolated acini from immature pancreas. Cholecystokinin (CCK) receptor density will be determined using 125I-Bolton-Hunter CCK 33 and receptor density will be correlated with secretory function at different ages. The role of nippling, enteral feeding and nutrition on development of pancreatic function will be established using the techniques of subcutaneous nipple ligation and total parenteral nutrition. The effects of hormones known to be trophic for immature pancreas, and these manipulations on DNA and protein synthesis will be determined. Ultrastructural correlations between developmental stages and changes in development induced by hormones will be identified.